1. Field of the Invention
The present invention relates to cores for winding webs of delicate material. More in particular, this invention relates to a method and a core which reduce core impressions in the winding of photographic film and paper.
2. Description of the Prior Art
Core impressions are a major source of waste when winding webs paper or film on cores which serve in the production process for winding such paper or film after its passage through coating and drying stations, and for its storage until it is unwound in order to be longitudinally slit and transversely cut so that small rolls or sheet stacks are obtained that fit in the end-user's exposure apparatus.
Core impressions, also called "first winding defects", are caused by the leading edge of a wound web which causes a pressure mark in the second winding of the web at the place where the latter is in contact with this leading edge. The contact pressure at such place is high since in the region of said edge the second winding is in supporting contact with the first winding on one side only of the leading edge, there being no first winding support at the other side. The increased local pressure on the second winding causes a distortion of such winding which on its turn affects the third winding, and this the still next one, etc.
The caused damage can be a surface defect, such as impressions or scratches, but usually it will be so-called pressure fog which is a local desensitization of (a) light-sensitive layer(s) as a consequence of increased mechanical pressure.
The mentioned defect extends in practice from a few up to 100 or even more windings of a roll of wound material, the satisfactory recovery of which is mostly excluded because of the number of differently sensitized and/or coloured layers involved.
It is disclosed in the art to provide the core with a covering of elastically or plastically deformable material which deforms to accomodate the leading edge so that the first turns of a web on a core do not have to deform to accomodate the irregularity caused by the leading web edge.
We have found that none of both materials provides entirely acceptable results in practice. The expression "elastically deformable" points to a relation between pressure and thickness of a layer and this means that, although the covering in question now allows the leading end of the wound web to (partially) enter the covering, there will be a local increased pressure on the next winding(s) caused by the compressed material, whereby the aimed improvement is only partly obtained.
In the case of a plastically deformable covering, it is theoretically possible that a core covering could be deformed in such a way as to completely accomodate a leading web end thus offer a uniform supporting pressure for the next web winding(s). However, this situation can occur only once since in a next winding operation the leading web edge will inevitably take another angular position whereby not only the leading web edge but the plastically deformed core as well will damage the next windings of the web.